Binocular vision, or the use of two eyes working in conjunction, can be achieved only with a well-developed, coordinated oculomotor and neural system and with the optical functioning of each eye in reasonable adjustment. When some of the mentioned mechanisms fail, binocular vision can be impaired, as it is described by Bennett & Rabbetts 1998, Clinical Visual Optics, 3rd edition.
Heterophoria and heterotropia are anomalies of a binocular vision and are conditions that prevent a person from directing both eyes simultaneously towards to a fixation target. These anomalies can be diagnosed by dissociating the eyes. In the case of heterophoria, for example, if a patient fixates a stationary fixation target and one of the eyes is covered (or dissociated, as it is called), the covered eye will turn by an angle a so that the visual axis no longer passes through the fixation target, as it is shown in panel A of FIG. 1. When the cover is removed, bifoveal fixation is rapidly regained, as shown in panel B of FIG. 1. This behaviour is described in Bennett & Rabbetts 1998, Clinical Visual Optics, 3rd edition.
In the case of heterotropia, patients do not achieve bifoveal fixation of any object and one of the eyes shows a manifested deviation, even without dissociation, as shown in panel A of FIG. 2. This is also called strabismus. In this case, if the strabismic eye is covered while the patient looks at the fixation point, neither the covered nor the uncovered eye will move, however if the originally fixating eye is covered, the strabismic eye will turn through the angle of misalignment and foveate the fixation point, as it is shown in panel B of FIG. 2. This behaviour is also described in Bennett & Rabbetts 1998, Clinical Visual Optics, 3rd edition.
A prevalence of strabismus ranging from 2% to 5% among preschool and school-aged European children has been found in some population-based studies (Frandsen A D. “Occurrence of Squint,” Acta Ophthalmol Suppl. 1960, 62, pages 1 to 158, and “Prevalence and risk factors for common vision problems in children: Data from the ALSPAC study,” Williams C, Northstone K, Howard M, Harvey I, Harrad R A, Sparrow J M, Br J Ophthalmol., July 2008, 92(7) pages 959 to 964) and similar values have been found in African American populations (Baltimore Vision Screening Project, Preslan M W, Novak A, “Ophthalmology,” January 1996, 103(1), pages 105 to 109, and Giordano L, Friedman D S, Repka M X, et al. “Prevalence of strabismus and amblyopia in preschool-aged children: The Baltimore Pediatric Eye Disease Study,” Invest Ophthalmol Vis Sci., 2008, 49, E-abstract 1552). In a study of Japanese school children, a prevalence of 1% was found (“The prevalence of strabismus and amblyopia in Japanese elementary school children”, Matsuo T, Matsuo C, Ophthalmic Epidemiol., February 2005 12(1), pages 31 to 36, and “Comparison of prevalence rates of strabismus and amblyopia in Japanese elementary school children between the years 2003 and 2005,” Matsuo T, Matsuo C, Acta Med Okayama, December 2007, 61(6), pages 329 to 334) and same prevalence of strabismus was found in a population-based sample of native American children in the Kindergarten and first-grade (Garvey 2010 “Prevalence of strabismus among preschool, kindergarten and first-grade Tohono O'Odham children”).
Usually, cover tests are performed to determine the presence, classification, and magnitude of an ocular deviation. The test classifies the heterophoria based on the movement of the eye under cover. The classification can be esophoria or exophoria for horizontal movements and hyperphoria or hypophoria for vertical movements. The classification of heterotropia is based on the relative position of the deviating eye and can be esotropia or exotropia for horizontal eye movements and hypertropia or hypotropia for vertical movements. Another method is the so-called MKH (“Mess- und Korrektur nach Hase”) measure and correction method which is critically discussed in the community of opticians and ophthalmologists.
Once a heterophoria or heterotropia is diagnosed, it is treated by refractive, prismatic, or orthoptic means. Also surgery is an option, typically the last one.
The current methods of manually diagnosing and treating heterophoria or heterotropia often have the following issues:
Diagnosis, classification and the determination of magnitude of heterophoria or heterotropia lead to a prescription that is verified with the standard optometric methods. This prescription, e.g., a prescription for prismatic glasses, is never tested in a realistic life environment before it is actually applied.
Diagnosis is subjective and based on the experience of the optometrist or examiner.
Determination of the magnitude of correction is estimated by subjective observation of the optometrist and, therefore, the accuracy of the correction also depends on the experience of the examiner.
Determination of oblique deviations is hard to be achieved and, thus, often uncorrected.
Strabismus testing apparatuses as they are described in U.S. Pat. No. 5,094,521 and in CN101147670 (A) can overcome some of these issues. In particular, U.S. Pat. No. 5,094,521 describes a measuring device that allows diagnosing strabismus and suggesting a treatment. However, even when using such testing apparatus, there is no opportunity for testing suggested prisms in a realistic life environment.
WO 2016/139662 A1 discloses a method and a device for measuring strabismus or heterophoria.
EP 3 109 694 A1 discloses a spectacle lens supply system in which a size and an area of clear vision in a progressive power lens is determined according to a level of prescribed fixation disparity amount.
EP 2 856 931 A1 discloses a simulation device for simulating the effect of a spectacle lens where, e.g., prismatic power is considered. In the context of the simulation, a stereoscopic image acquired by imaging cameras is processed to create a simulation image which is supposed to be viewed by the spectacle wearer through the spectacle lens.